My theory is the E-D
originally was supposed to be a deep-range explorer, pushing the boundaries of the Federation and making first contact with more advanced civilizations, but after the Yamato
's destruction, Starfleet reevaluated the whole Galaxy
-class concept and decided ships full of civilians shouldn't be boldly going. That explains the change of tone from the early seasons, when the Enterprise
seemed to be on the edge of known space, to later years when they were ferrying supplies around, investigating missing ships, and making contact with less advanced civilizations.